add vcycle command

Command/VCycle.hs

@@ -0,0 +1,45 @@
+{- git-annex command
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Command.VCycle where
+
+import Common.Annex
+import Command
+import Annex.View
+import Types.View
+import Logs.View
+import Command.View (checkoutViewBranch)
+
+def :: [Command]
+def = [notBareRepo $ notDirect $
+	command "vcycle" paramNothing seek SectionUtility
+	"switch view to next layout"]
+
+seek :: CommandSeek
+seek = withNothing start
+
+start ::CommandStart
+start = go =<< currentView
+  where
+	go Nothing = error "Not in a view."
+	go (Just v) = do
+		let v' = v { viewComponents = vcycle [] (viewComponents v) }
+		if v == v'
+			then do
+				showNote "unchanged"
+				next $ next $ return True
+			else next $ perform v'
+
+	vcycle rest (c:cs)
+		| multiValue (viewFilter c) = rest ++ cs ++ [c]
+		| otherwise = vcycle (c:rest) cs
+	vcycle rest c = rest ++ c
+
+perform :: View -> CommandPerform
+perform view = do
+	branch <- narrowView view
+	next $ checkoutViewBranch view branch